By custom and practice, the electromagnetic spectrum between the wavelengths of the vacuum ultraviolet at 0.001 m and the far infrared at 100 m. Note: The term "optical spectrum" originally applied only to that region of the electromagnetic spectrum visible to the normal human eye, but is now considered to include all wavelengths between the shortest wavelengths of radio and the longest of x-rays. At this writing, no formal spectral limits are recognized nationally or internationally.